I have a UDT that contains an instance of a "SubUDT" who's definition is inherited from a UDT that's inherited from a UDT... so on.
i.e. path to this UDT instance is
UDT / SubUDT_Instance < UDT_Parent1 < UDT_Parent2 < UDT_Parent3 < UDT_Parent4
What has happened here is that updates to the definition of this UDT do not automatically populate to instances of the UDT. If a new instance (exact replica, just created after the definition is updated) is created, this instance is updated, and can even coexist with the outdated instance.
I have verified this behavior with support and I am not aware of a workaround at the moment, other than restart each instance of the UDT (you can also restart the parent folder that holds the instance).